Superuser can't log in after changing 'trust' in pg_hba.conf

I am setting up a new postgreSQL/Linux installation (latest stable pg.)
After changing the line in the pg_hba.conf file:

local all trust

to read:

local all md5

I was no longer able to access the database with psql as 'postgres'. I left
the other line in this default implementation of the file alone (i.e. the
host line.) The password I gave was correct but was not accepted by the db.
Other users' passwords were accepted with no problem. The postgres user's
password was tied to the Linux system password for that account. I tried
with a login (su postgres) and the password was accepted there. So then I
reset the pg_hba.conf file, logged in with psql and changed the password
with ALTER USER. Then I reset the pg_hba.conf again to read 'md5'
encryption and now I can login with 'postgres' user. Can anyone tell me why
this is? Also, since it took a short while of dancing around to figure this
out, should this appear as a doc note in the instructions?

Also, I am forced to restart the postmaster to get it to begin challenging
for passwords when the documentation clearly says this isn't necessary. Is
this known as well? Again should I create a doc note for this?